The present invention relates generally to apparatus and their use for surface treatments using plasma assisted processing and more particularly, but not exclusively, for the treatment of large flat substrates.
Such treatments can include etching, deposition, cleaning, passivation and ion implantation.
The new requirements for the plasma processing of large substrates become more and more critical for plasma sources available on the market. The success of the plasma assisted processing depends on the scalability of these plasma sources.
To fulfill these requirements, new plasma sources must be envisaged to process large substrates with plasma features like the generation of high densities of reactive species with low and controllable energy over a wide pressure range, and with an excellent homogeneity throughout the substrate.
Plasma processing generally uses a vacuum chamber connected to a gas inlet and a pumping device for controlling the gas flows and pressure. Electrical energy is applied to the vacuum chamber to accelerate the free electrons in the gases to the energy of ionization of the gas molecules, which thereby creates plasma. Ionization phenomena free some electrons which can also be accelerated to the ionization energy.
The added energy of the free electrons in the gas is generally accomplished by an electric field, a varying magnetic field, or both.
One traditional method used in plasma processing to generate plasma is by a technique known as Capacitively Coupled Plasma. The plasma results from the application of an AC voltage between two electrodes creating an electric field which accelerate the free electrons. Generally, one of the two electrodes is the substrate holder. The applied energy generated by the AC voltage applied between the two electrodes controls at the same time the flux and kinetic energy of the ions. Because the two parameters are coupled, this process is difficult to optimize.
Another plasma source used in plasma processing is based on the Electron Cyclotron Resonance (ECR). In this process, microwave power is applied to the gas together with a constant magnetic field which transforms the electron paths into a circular path. The intensity of the magnetic field is such that the frequency of gyration of the electron is the same as the frequency of the electric field, which leads to a resonance effect increasing the efficiency of electron acceleration. This type of excitation mode can provide a plasma with high ion flux and low ion energy. The ion energy can be controlled by applying an independent bias to the substrate. However, such an apparatus is complex and expensive. Moreover, it is still too limited as regards the plasma expected processing expected features, in particular for scaling up and homogeneity of the plasma source.
A new generation of plasma source has been developed during the last years which give good promise. These are known as Inductively Coupled Plasmas (ICPs), such as described e.g. in U.S. Pat. Nos. 4,948,458 and 5,277,751. The plasma is created by a varying magnetic field generated by a spiral planar coil. The electrons are accelerated in a circular path parallel to the coil plane and the insulating window of the plasma chamber. This configuration provides a high density plasma with low kinetic energy, but has an inherent problem of homogeneity at the center and at the boundary of the coil when the size of the apparatus is increased. This problem limits the scability of the process.
U.S. Pat. No. 5,435,881 presents an apparatus for generating a suitably low pressure planar plasma. This apparatus comprises a two-by-two or a larger array of alternating magnetic poles (multipoles). The advantages cited in this patent are the possibility to generate a large plasma by adding more varying magnetic poles, therefore having very small area on non uniform plasma.
However, such a design creates a dependency between the spacing of the two-by-two magnetic poles and the excitation frequency as well as the in-use operation pressure. This spacing depends on the mean free path of the electrons which decreases when the pressure increases. Accordingly, when a high operating pressure is necessary for process requirements, the spacing between the two-by-two poles must be drastically decreased. This becomes critical from a technical point of view. The process also requires different multipole distributions for different process pressures, which decreases its flexibility and applicability to industrial applications.
In all these prior art apparatus, there is a problem of gas distribution uniformity in the chamber center. The gas distribution is usually made using a ring located in the side wall of the plasma chamber, which results in a lack of gas distribution uniformity at the chamber center. This non-uniformity is even more acute when the plasma chamber dimension increases. Moreover the gas distribution means are usually made of metallic material, which perturbs the magnetic field inside the chamber, and thus the plasma density.
Document EP-776 645 apparently discloses a plasma reactor or plasma chamber in which a uniform gas distribution is achieved across a wafer surface by injecting gas through a center gas feed silicon or semiconductor ceiling.
This device is schematically illustrated on FIG. 1, and comprises a plasma chamber 2, covered by a semiconductor ceiling 6 through which gas injection tubes 12, 14 are drilled. Tube 14 in turn is connected to a center gas feed pipe 16.
An overhead inductive coil antenna 4 is held in an insulating antenna holder 8 connected to a plasma source power generator through an impedance match circuit 10.
In this device, a voltage of about 2000 to 3000 volts is usually applied to the coil antenna. A correspondingly very high electric field can thus be induced in the dielectric window constituted by the semiconductor ceiling 6. Such a capacitive coupling is very detrimental.
This document further suggests choosing either a dielectric or semiconductor, as a material for the top ceiling. However, dielectric or semiconductor material results in a plasma being created in tubes 12, 14, because of this capacitive coupling, which is gas consuming and can damage the semiconductor ceiling.
The invention concerns an apparatus for generating a time-varying magnetic field in a plasma processing chamber to create or sustain a plasma within the chamber by inductive coupling, characterised in that it comprises:
a magnetic core presenting a pole face structure or a unipolar pole face structure
an inductor means associated with the magnetic core, for generating a substantially uniformly distributed time-varying magnetic field throughout the pole face or unipolar pole face structure,
means for injecting gas into said chamber and through said magnetic core.
Since the means for injecting gas into the plasma chamber are located or inserted through said magnetic core, a uniform or controlled gas distribution is achieved in a plasma processing chamber having such an apparatus for generating a time-varying magnetic field, without any perturbation of the magnetic field.
Furthermore, the magnetic core electrostatically isolates the means for gas injection from the inductor means. In other words, the magnetic core plays the role of an electrostatic screen between the means for gas injection and the inductor means, thus eliminating the risk of capacitive coupling. The risk of plasma induction in the gas injecting means themselves is reduced.
According to one embodiment of the invention, said means for injecting gas into said chamber form a showerhead-like gas injection.
For example, they advantageously comprise a plurality of injection pipes distributed through the magnetic core. These injection pipes are made of stainless steel material, or of an insulating material.
An advantage of this embodiment is that the number of injection pipes can be adapted without perturbing the magnetic field. In other words, the number of pipes does not influence the magnetic field inside the plasma chamber.
The diameter of the pipes can also be varied in a same magnetic core. More gas is injected through larger pipes, than through comparatively smaller pipes. It is thus possible to achieve a controlled gas injection in the plasma chamber.
The injection pipes are connected to gas distributing means for distributing gas to the injection pipes.
These gas distributing means are preferentially located on the side of the magnetic core opposite to an inner space of said plasma processing chamber
In one embodiment, they comprise a common gas injection pipe, through which gas is distributed to the injection pipes. This common gas injection pipe is preferentially made of stainless steel, in particular in case of corrosive gases.
In another embodiment, the gas distributing means comprise a cover, located on the side of the magnetic core opposite to the inner space of the plasma processing chamber with a gap between said cover and said magnetic core, said injection pipes emerging in said gap.
A gas, or gases, is/are mixed in the gap between the cover and the magnetic core, thus increasing the homogeneity of the gas distributed or injected in the inner space of the plasma chamber. The gap thus forms a gas distribution area above the magnetic core.
Moreover, this arrangement avoids the connection of any gas distribution pipe (the above mentioned stainless steel common gas injection pipe) to the magnetic pole.
Preferably, the unipolar face structure is constituted by a single pole face of unitary construction. In this way, the plasma processing chamber is confronted with a substantially continuous surface, which further contributes to enhance uniformity.
It is nevertheless conceivable to divide the pole face structure into two or more pole faces or unipolar pole faces that confront respective portions of the plasma processing chamber. This alternative solution may be considered if the area to be covered by the magnetic core is particularly large. The pole faces may then be associated to respective inductors and power supplies whilst being kept in phase to ensure that all the pole faces have the same polarity at any one time. The pole faces may alternatively physically depend from a common magnetic core and inductor.
In a preferred construction, the pole face structure constitutes an end face of the magnetic core.
Advantageously, the magnetic core comprises at least one electrical discontinuity in a path along a plane parallel to the pole face so as to prevent the circulation of eddy currents around the core. Indeed, the magnetic flux lines passing through the magnetic core tend to create eddy currents that circulate in the plane of the pole face, by Lenz""s law. If these currents were free to circulate around the core, they would create magnetic flux lines that oppose those generated by the coil, with the effect of diminishing the net magnetic field energy emitted from the pole face, and of creating undesirable heating of the core.
The discontinuity can be in the form of one or more laminations. The lamination(s) preferably extend radially from a point proximal to or at the centre of the core to the to the periphery thereof. The laminations may occupy the entire depth of the magnetic core, as measured in the direction perpendicular to the pole face structure.
The above problem of eddy currents is more pronounced in some core designs than in others depending, for instance, on the core material used, and on the operation frequency, and it may not always be necessary to provide such a discontinuity.
The inductor means typically comprises a conductor arranged to form one or more turns around at least a portion of the magnetic core. It may be wound around the periphery of the magnetic core. The inductor means may also comprise a planar winding recessed within a groove pattern formed in the magnetic core, e.g. at the pole face surface.
The inductor means is driven by a power supply preferably delivering current at a frequency of around 10 kHz to 100 MHz, a typical operating frequency being 13.56 MHz. A circuit for impedance matching and phase factor correction can be provided between the power supply and the inductor if required.
The invention also concerns a plasma processing apparatus comprising:
a plasma processing chamber having at least one field admission opening or window
at least one magnetic field generating apparatus as defined above, arranged to create a time-varying magnetic field in the chamber,
power source means for driving the magnetic field generating apparatus.
A barrier can be formed between a field emission surface and the plasma environment in order to prevent that surface from contaminating the chamber.
Such a barrier comprises a sheet of dielectric material maintained between said pole face structure and an inner space of said plasma processing chamber.
Alternatively, the barrier comprises at least one field admission window between an inner space of said plasma chamber and said magnetic field generating apparatus. In this case, the means for injecting gas into the chamber and through said magnetic core traverse the window.
In this case, said magnetic core presents a unipolar face adapted to be applied against or in proximity to the window.
The magnetic core can easily be matched to the shape and dimensions of an opening or of a window of the plasma chamber; it can present e.g. a circular, rectangular or polygonal pole face as required.
A window of the processing chamber need not necessarily be flat, but may be curved, e.g. to follow the contour of a wall portion from which it/they depend(s). The magnetic core can likewise present a non planar pole face structure configured to follow the curvature of the window(s) to provide uniform conditions inside the chamber.
In the case of injection pipes, each of said injection pipes traverses said window through a corresponding hole in said window, and an end of each of said injection pipes is welded to the periphery of said corresponding hole.
The plasma processing chamber may comprise several field-admission windows. For example, it can be provided with two oppositely-facing windows. If the chamber has a shallow configuration (circular or square cross-section), the windows may be provided at each end of the shallow walls, for example. If the chamber has an elongate configuration (circular or square cross-section) the windows may be formed on the elongate walls, e.g. disposed in one or several pair(s) of oppositely-facing windows.